1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, and a facsimile machine.
2. Description of the Related Art
Hitherto, in an electrophotographic color image forming apparatus in which a toner image is formed on a photosensitive drum (an image bearing member) to be sequentially transferred in layers to an intermediate transfer drum and the image on the transfer drum in turn is further transferred to a transfer material such as a sheet of paper by a transfer member such as a transfer roller and a transfer belt capable of engaging and separating from the intermediate transfer drum, when color printing is continuously controlled, the pressure balance of an entire image forming mechanism is changed by a transfer action of the transfer member on a transfer material. This results in a positional deviation of the transfer member and an image-deteriorating problem such as color mis-registration and banding owing to vibration generated during the abutting and separating of the transfer member, which affects the next image. Referring to FIGS. 2 and 3, the contents will be further described. As for each of the members and mechanisms in FIGS. 2 and 3, the details will be described in the item of the preferred embodiments referring to FIG. 1. FIG. 2 is a schematic view illustrating the abutting state of the transfer roller in intentional exaggeration. Referring to FIG. 2, when the transfer roller 10 moves toward the intermediate transfer drum 9 (in the direction of the arrow) and abuts the intermediate transfer drum 9, a force is applied to the intermediate transfer drum 9 in the direction of the arrow. Simultaneously, a force in the direction of the arrow is also applied to the photosensitive drum 15 abutting the other side of the intermediate transfer drum 9, such that the position of a laser beam applied to the photosensitive drum 15 from a scanner unit 30, that is the position "P" at which a latent image is formed, will become slightly mis-aligned. Therefore, the latent image forming position "P" and a primary transfer position "Q" changes slightly according to whether the transfer roller 10 is abutting the intermediate transfer rollers or not, so that the distance between the latent image forming position changes and the transfer position along the peripheral surface.
The effect on the next image will be described in timing progress by referring to FIG. 3. The signals shown in FIG. 3 will be described in detail by referring FIG. 5. AS will be understood from FIG. 3, while the transfer roller is driven, that is, at "on" timings of the T2R" signal, which is the driving signal of the transfer roller, the next "Y" latent image is adversely affected during forming of this image.
Therefore, during forming of continuous images, the toner image (the latent image) formed when the second color is mis-registered relative to the toner image (the latent image) of the first color when the second page is formed on the photosensitive drum, resulting in producing color mis-registration. This phenomenon occurs in a like manner when an "ICL" roller 39, which charges the residual toner on the intermediate transfer drum with electricity, abuts on and is separated from the intermediate transfer drum.
Conventionally, as will be described by referring to FIG. 5, a rotation without image forming of the photosensitive drum 15, that is idle rotation, is interposed every time during the driving of the transfer roller while forming continuous images in order to minimize color mis-registration and banding by avoiding adverse effects of the transfer roller 10. This is a passive countermeasure that avoids the impact of the mechanical structure and waits for convergence of the vibration rather than a positive measure that eliminates the effect.
In the above-described conventional examples, during forming of continuous images, there has been the problem of color mis-registration or reduced throughput due to every time interposition of the above-described idle rotation.